DESCRIPTION (the applicant's description verbatim): The insulin-like growth factor 2/mannose 6-phosphate (IGF2/M6P) receptor is a multi-functional protein with distinct binding sites for diverse ligands including IGF2, M6P, urokinase-type plasminogen activator receptor and retinoic acid. The IGF2/M6P receptor is clearly critical for regulating heart cell number during embryogenesis. IGF2/M6P receptor-null embryos die before birth with hyperplastic (increased cell number) hearts. What is not clear is how the receptor regulates cardiac cell number. Receptor-null hyperplasia does not appear to be due to excessive proliferative activity; this implies that cell death is subnormal in IGF2/M6P receptor-null hearts. Thus, we hypothesize that the IGF2/M6P receptor regulates cell number by modulating the levels of extracellular growth factors that affect cardiac cell survival and death. The proposed work will focus on determining whether two growth factors known to interact with the IGF2/M6P receptor, IGF2 and transforming growth factor beta (TGFbeta), are important for the development of cardiac hyperplasia in receptor-null mice. This will be critically examined in a series of experiments. First, we will use a combination of biochemical and microscopic techniques to thoroughly compare the incidence of cell death in receptor-null and control hearts. Second, we will perform a careful analysis of cardiac growth in mice that lack both IGF2 receptor and ligand to determine if the IGF2/M6P receptor regulates cardiac growth in an IGF2-independent manner. Third, we will examine whether expression and/or activation of TGFbeta are altered in IGF2/M6P receptor-null embryos. Finally, we will attempt to rescue the cardiac phenotype in IGF2/M6P receptor-null embryos by increasing endogenous levels of active TGFbeta. The proposed experiments will provide insight into the molecular basis of myocardial growth regulation during embryogenesis. Abnormal growth is associated with several congenital heart defects as well as the transition to congestive heart failure in adults. A better understanding of how myocardial growth is regulated may ultimately lead to clinical approaches for preventing both of these health problems.